The present invention is directed to ultra-high aspect ratio, hierarchical nanocapillary arrays and the like and a method for fast fabrication thereof, namely by combining temperature, voltage and acid concentration controlled electrochemical oxidation of aluminum as well as the growth of materials within each nanocapillary, such as carbon nanotubes through catalytic chemical vapor deposition. These hierarchical nanocapillary arrays can be either fabricated freestanding or fabricated directly on support materials such as electrically conductive or insulating material. Furthermore, inclusion of material within the hierarchical nanopores facilitates the directed fabrication of ultra-high aspect ratio and density nanocapillary arrays.
High-density hydrogen storage that can be safely transported and delivered still presents a major bottleneck in the shift from a hydrocarbon-based to a hydrogen-based energy infrastructure and as such, has become an area of intense research. Capillary-based storage is effective due to the high pressure tolerance of the glass microcapillaries as well as the fact that only a portion of the total contained hydrogen is stored in each capillary. As a result, rupture of the container would not result in a highly energetic release of all of the gas.
Hydrogen storage densities in glass microcapillaries are limited by the internal pressures that can be sustained by the microcapillaries. These pressures rely on the tensile strength of the glass, and this microcapillary fabrication technique is not likely to be able to be adapted for higher tensile strength materials. The natural surface area of these microcapillary arrays is low, and thus, storage enhancements through adsorption could only be obtained through impregnation of the microcapillaries with an absorbent, resulting in severely diminished volumetric storage.
A natural extension of this proven, commercialized technology is the construction of nanocapillary arrays through porous templates such as anodic aluminum oxide (AAO). Nanocapillary arrays enhance the storage pressure capabilities of microcapillaries through capillary diameter reduction and also produce massive surface areas, allowing for additional physical hydrogen storage through adsorption. The pore wall thickness to diameter ratio (which determines pressure tolerances) is highly “tunable”, and the pore wall can be coated with a high tensile strength material, enabling maximum gravimetric (Gc) and volumetric (Vc) densities. If the pore wall is included with a higher tensile strength material such as carbon nanotubes (CNTs), the pressure tolerance of the pore can be increased by over 20 times that of a similarly sized glass capillary. In addition, CNT growth in microcapillaries is not possible due to the diameter of the capillary. CNTs also adsorb hydrogen; which, combined with a 100,000-fold increase in surface area from the AAO, physical storage through enhanced compression in nanocapillaries and CNT-hydrogen adsorption can drastically enhance this safe, proven method of storage with a CNT/AAO nano-material hybrid structure. Hydrogen adsorption has been shown to be as high as 15% by weight and projected to be up to 100% in CNTs. [Züttel, Sudan, Mauron, Kiyobayashi, Emmenegger and Schlapbach; Hydrogen storage in carbon nanostructures. Int. J. Hyd. En., 2002, (27), 203-212.] Gas compression in nanocapillaries is expected to achieve capillary condensation at low pore diameters. This results from the surface curvature/energy of the nanocapillary and should enhance storage density as well as reduce the energy required to pressurize the nanocapillary array.
AAO nanocapillary arrays can be fabricated with a total diameter (overall storage container diameter) of virtually any size, making the self-assembly fabrication process highly scalable. The individual pores can range from 10 to 500 nm, with pore densities ranging from 108 to 1012 pores/cm2. The wall thickness of each individual pore, as fabricated, is typically the same as the pore radius. However, the wall thickness can be decreased by simple etching in a dilute acid or increased by coating of the pore wall or growth of a nanotube within the pore (such as CNTs).
In a currently preferred embodiment of our invention, these high aspect ratio nanocapillary arrays can be used for compression and adsorption-based gas storage for materials such as hydrogen, gas, krypton, xenon and the like. In addition, the nanocapillary arrays can be incorporated with materials that facilitate electrochemical gaseous compression of certain gases by electrolyzing and subsequently transporting low-pressure gas outside of the nanocapillary into its interior, with the gas being pressurized within each nanocapillary. Our process of applying high potentials at low temperatures results in the fast fabrication of conformal (or conformable) hierarchical nanocapillary arrays which are grown on any shaped support so long as the initial support is composed of aluminum, zinc, tin, antimony, titanium, magnesium, niobium, tantalum or any other metal that undergoes electrochemical formation of ordered nanopores. The fabrication methods of the present invention result in more than a 100-fold advance over the state-of-the-art techniques.
The materials listed are known producers of arrayed nanopores under anodic electrochemical oxidation (abbreviated herein forth as “anodization” or “anodized”). In other contemplated embodiments these nanocapillary arrays can be leveraged in applications where ultra-high surface area, vertically oriented and arrayed nanocapillaries are desired such as in batteries, capacitors, electrochemistry, chemical conversion, photovoltaic devices as well as many other chemical and physical applications. Our technology is not only applicable to hydrogen as a gas storage media, but also other gases such as xenon and krypton (for applications in ion propulsion systems for next-generation spacecraft) or gas (for self-contained breathing apparatuses or breathing gas). In addition to gaseous storage, the developed nanomaterial can be used to increase charge collection efficiency in nanostructure photovoltaics, increase energy density in supercapacitors as well as provide ultra-high surface area scaffolding for catalysis and thermal energy storage.